Nano-structured refractory metals, metal carbides, and coatings and parts fabricated therefrom

ABSTRACT

Refractory metal and refractory metal carbide nanoparticle mixtures and methods for making the same are provided. The nanoparticle mixtures can be painted onto a surface to be coated and heated at low temperatures to form a gas-tight coating. The low temperature formation of refractory metal and refractory metal carbide coatings allows these coatings to be provided on surfaces that would otherwise be uncoatable or very difficult to coat, whether because they are carbon-based materials (e.g., graphite, carbon/carbon composites) or temperature sensitive materials (e.g., materials that would melt, oxidize, or otherwise not withstand temperatures above 800° C.), or because the high aspect ratio of the surface would prevent other coating methods from being effective (e.g., the inner surfaces of tubes and nozzles). The nanoparticle mixtures can also be disposed in a mold and sintered to form fully dense components.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of pending: U.S. patentapplication Ser. No. 13/891,597, filed on May 10, 2013, now U.S. Pat.No. 8,753,720, which is a divisional of U.S. patent application Ser. No.12/191,975, filed on Aug. 14, 2008 and now abandoned, each of which isincorporated herein by reference in its entirety for all purposes andclaims the benefit of priority under 35 U.S.C. §119 from U.S.Provisional Patent Application Ser. No. 61/017,098 entitled “FORMATIONOF NANO-STRUCTURED REFRACTORY CARBIDE COATINGS AND PARTS,” filed on Dec.27, 2007, the disclosure of which is hereby incorporated by reference inits entirety for all purposes. The present application is also relatedto U.S. patent application Ser. No. 11/798,529, entitled “RHENIUMNANOPARTICLES” filed on May 15, 2007, now U.S. Pat. No. 7,736,414 thedisclosure of which is also hereby incorporated by reference in itsentirety for all purposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

FIELD OF THE INVENTION

The present invention generally relates to nano-structured refractorymetals, metal carbides and, in particular relates to methods and systemsfor manufacturing nano-structured refractory metals, metal carbides, andcoatings and parts.

BACKGROUND OF THE INVENTION

Advanced hot-gas control systems are being designed and tested withoperating temperatures in excess of 3000° F. These operatingtemperatures preclude the use of many metals in the construction, of thecontrol systems. Refractory metals and their carbides, however, havevery high melting points (e.g. TiC @ 3140° C., NbC @ 3500° C., ZrC @3540° C., TaC @ 3880° C. HfC @3890° C. and the mixed phase Ta₄HfC₅@4215°C.), and (3632° F.). Moreover, refractory carbides exhibit, unusuallyhigh oxidation resistance and can therefore be used in structuralapplications to temperatures exceeding that of refractory metals such aslie. Refractory carbides are also significantly lighter than refractorymetals, with densities around 5-12 g/cm (by way of comparison, Re, Irand W have densities around 19-22 g/ccm).

Refractory metal and refractory metal carbide components are, however,difficult to produce using metallurgical processes such, as casting,forming, machining, and joining. Because of these metals' high meltingpoints, casting may be impractical, and therefore powder metallurgy isthe primary process for producing refractory metal plates or barstock.This process is labor intensive, expensive, and has a long lead time, ascomponents made via powder metallurgy must go through multipleprocessing steps and heat treatments, followed by costly and laboriousmachining processes that require special equipment.

SUMMARY OF THE INVENTION

The present invention solves the foregoing problems by providing amethod for producing refractory metal and refractory metal carbidecoatings and components from refractory metal and/or refractory metalcarbide nanoparticles. Such nanoparticles can be used in manyapplications, including, for example, in the low-temperature formationof fully dense parts. In this regard, a refractory metal or refractorymetal carbide nanoparticle mixture can be disposed in a mold andsintered to form fully-dense refractory metal or refractory metalcarbide-components with enhanced microstructures (e.g., providing highfracture toughness and enhanced ductility). The nanoparticles enjoy awide range of uses, beyond fully dense part fabrication. For example, arefractory metal or refractory metal carbide nanoparticle mixture can bepainted onto a surface to be coated and heated to form a gas-tightcoating. The low temperature formation of refractory metal andrefractory metal carbide coatings allows these coatings to be providedon surfaces that would otherwise be uncoatable, whether because of thetemperature requited to perform hot isostatic pressing (“HIP”) on acoating or because of the high aspect ratio of the surface which wouldprevent chemical vapor deposition from being effective). Thenanoparticles may be formed by reacting refractory metal precursors inthe presence of various surfactants that can limit the growth of thenanoparticles to particular sizes or ranges of sizes.

According to one embodiment of the present invention, a method formanufacturing refractory metal nanoparticles comprises the steps ofproviding a solvent, providing a refractory metal precursor including arefractory metal and one or more additional elements, providing areactant for reacting with the refractory metal precursor to free therefractory metal from the one or more additional elements, providing asurfactant, and combining the refractory metal precursor, the reactantand the surfactant in the solvent to form a plurality of refractorymetal nanoparticles and to surround each refractory metal nanoparticlewith a layer of molecules of the surfactant.

According to another embodiment of the present invention, a method forforming a refractory metal coating comprises the steps of providing arefractory metal nanoparticle mixture including a solvent and aplurality of refractory metal nanoparticles, each of the plurality ofrefractory metal nanoparticles being surrounded by a layer of surfactantmolecules, disposing the refractory metal nanoparticle mixture on asurface to be coated, heating the refractory metal nanoparticle mixtureto a first temperature to evaporate the solvent and leave the pluralityof refractory metal nanoparticles surrounded by surfactant molecules onthe surface, heating the refractory metal nanoparticles and thesurfactant molecules to a second temperature to remove the surfactantmolecules and leave the plurality of refractory metal nanoparticles onthe surface, and heating the refractory metal nanoparticles to a thirdtemperature to bond the refractory metal nanoparticles to form a coatingon the surface.

According to yet another embodiment of the present invention, a methodfor forming a refractory metal carbide coating comprises the steps ofproviding a refractory metal nanoparticle mixture including a solventand a plurality of refractory metal nanoparticles, each of the pluralityof refractory metal nanoparticles being surrounded by a layer ofsurfactant molecules, disposing the refractory metal nanoparticlemixture on a surface to be coated, heating the refractory metalnanoparticle mixture to a first temperature to evaporate the solvent andleave the plurality of refractory metal nanoparticles surrounded bysurfactant molecules on the surface, and heating the refractory metalnanoparticles and the surfactant molecules to a second temperature todecompose the surfactant molecules and to react the plurality ofrefractory metal nanoparticles with carbon from the decomposedsurfactant to provide a refractory metal carbide coating on the surface.

According to yet another embodiment, of the present invention, a methodfor manufacturing refractory metal carbide nanoparticles comprises thesteps of providing a refractory metal alkyl precursor comprising arefractory metal and an alkyl group, and heating the refractory metalalkyl in the presence of a surfactant to decompose the refractory metalalkyl precursor into a refractory metal carbide nanoparticle surroundedby a layer of molecules of the surfactant.

According to yet another embodiment of the present invention, a methodfor forming a refractory metal carbide coating comprises the steps ofproviding a refractory metal carbide nanoparticle mixture including asolvent and a plurality of refractory metal carbide nanoparticles, eachof the plurality of refractory metal carbide nanoparticles beingsurrounded by a layer of surfactant molecules, disposing the refractorymetal carbide nanoparticle mixture on a surface to be coated, heatingthe refractory metal carbide nanoparticle mixture to a first temperatureto evaporate the solvent and leave the plurality of refractory metalcarbide nanoparticles surrounded by surfactant molecules on the surface,heating the refractory metal carbide nanoparticles and the surfactantmolecules to a second temperature to remove the surfactant molecules andleave the plurality of refractory metal carbide nanoparticles on thesurface, and heating the refractory metal carbide nanoparticles to athird temperature to bond.

According to yet another embodiment of the present invention, a methodfor forming a refractory metal component comprises/the steps ofproviding a refractory metal nanoparticle mixture in a mold, the mixtureincluding a solvent and a plurality of refractory metal nanoparticles,each of the plurality of refractory metal nanoparticles being surroundedby a layer of surfactant molecules, and sintering the refractory metalnanoparticle mixture to consolidate the refractory metal component.

According to yet another embodiment of the present invention, a methodfor harming a refractory metal carbide component comprises the steps ofproviding a refractory metal carbide nanoparticle mixture in a mold, themixture including a solvent and a plurality of refractory metal carbidenanoparticles, each of the plurality of refractory metal carbidenanoparticles being surrounded by a layer of surfactant molecules, andsintering the refractory metal, carbide nanoparticle mixture toconsolidate the refractory metal carbide component.

According to yet another embodiment of the present invention, a methodfor forming a refractory metal carbide component comprises the steps ofproviding a refractory metal nanoparticle mixture including a solventand a plurality of refractory metal nanoparticles, each of the pluralityof refractory metal nanoparticles being surrounded by a layer ofsurfactant molecules, disposing the refractory metal nanoparticlemixture in a mold, heating the refractory metal nanoparticle mixture toa first temperature to evaporate the solvent and leave the plurality ofrefractory metal nanoparticles surrounded by surfactant molecules in themold, and heating the refractory metal nanoparticles and the surfactantmolecules to a second temperature to decompose the surfactant moleculesand to react the plurality of refractory metal nanoparticles with carbonfrom the decomposed surfactant to provide a refractory metal carbidecomponent.

It is to be understood that, both the foregoing summary of the inventionand the following detailed description are exemplary and explanatory andare intended to provide farther explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide fartherunderstanding of the invention and are incorporated in and constitute apan of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention. In the drawings:

FIG. 1 is a flowchart illustrating a method for manufacturing refractorymetal nanoparticles in accordance with one embodiment of the presentinvention;

FIG. 2 is a flowchart illustrating a method for manufacturing refractorymetal nanoparticles in accordance with one embodiment of the presentinvention;

FIG. 3 illustrates a reactor used in the manufacture of refractory metalor refractory metal carbide nanoparticles in accordance with oneembodiment of the present invention;

FIG. 4 illustrates a nanoparticle surrounded with surfactant moleculesin accordance with one embodiment of the present invention;

FIG. 5 illustrates a refractory metal nanoparticle mixture in accordancewith one embodiment, of the present invention;

FIG. 6 is a flowchart illustrating a method for manufacturing refractorymetal nanoparticles in accordance with one embodiment of the presentinvention;

FIG. 7 is a flowchart illustrating a method, for forming a refractorymetal or refractory metal carbide coating, in accordance with oneembodiment of the present invention;

FIG. 8 illustrates the formation of a refractory metal or refractorymetal carbide coating in accordance with one embodiment of the present,invention;

FIGS. 9A, 9B and 9C illustrate the formation of a refractory metal orrefractory metal carbide coating in accordance with one embodiment ofthe present invention;

FIG. 10 is a flow chart illustrating a method, for forming a refractorymetal carbide coating in accordance with one embodiment of the presentinvention;

FIG. 11 is a flow chart illustrating a method for forming refractorymetal or refractory metal carbide components in accordance with oneembodiment of the present invention; and

FIG. 12 is a flow chart illustrating a method for forming refractorymetal carbide components In accordance with one embodiment of thepresent invention

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, numerous specific details are setforth to provide a full understanding of the present invention. It willbe apparent, however, to one ordinarily skilled in the art that thepresent invention may be practiced without some of these specificdetails. In other instances, well-known structures and techniques havenot been shown in detail to avoid unnecessarily obscuring the presentinvention.

I. Refractory Metal Nanoparticles

FIG. 1 is a flowchart illustrating a method for manufacturing refractorymetal or refractory metal nanoparticles in accordance with oneembodiment of the present invention. In step 101, a solvent is provided.In accordance with one aspect of the present invention, the solvent maybe a poly-ether solvent, such as, for example, a solvent characterizedby the chemical formula R—O—(CH₂CH₂—O)_(x)—R where x is a positiveinteger, and R is a methyl group (CH₃), an ethyl group (C₂H₅), a propylgroup (C₃H₇), or a butyl group (C₄H₆). Alternatively, the solvent may betriglyme or tetrahydrofruan (THF). In step 102, a refractory metalprecursor including a refractory metal of interest is provided. Forexample, in accordance with, one embodiment of the present invention,the precursor may be a chloride of the desired refractory metal such asTiCl₄. In step 103, a surfactant is provided. The surfactant may be anorganic amine, phosphine, acid or the like. For example. In the presentexemplary embodiment, the surfactant may be n-hexylamine (CH₃(CH₂)₅NH₂),n-nonylamine (CH₃(CH₂)₈NH₂), n-dodecylamine (CH₃(CH₂)₁₁NH₂), or anyother amine characterized by the chemical formula CH₃(CH₂)_(x)NH₂, wherex is a positive integer. In step 104, a reactant capable of reducing therefractory metal in the precursor to its atomic form is provided. Forexample, in the present exemplary embodiment, the reactant may besodium, naphthalene or lithium naphthalene, or any other reactantcapable of reducing TaCl₅. In other embodiments in which otherprecursors are used, other reactants may be used. For example, invarious other embodiments, the precursor may be any one of anyrefractory metal halide, fluoride, chloride, bromide, iodide, alcoxideor refractory metal complex with, acetylacetonate ligands, and thereactant may be any one of Li, K, Na, Mg, or Ca may be dissolved in THFand/or glyme in the presence of a promoter such as naphthalene (C₁₀H₈),anthracene (C₁₄H₁₀) or phenanthrene (C₁₄H₁₀).

In step 105, the precursor, the reactant and the surfactant are combinedin the poly-ether solvent to initiate a chemical reaction which formsnanoparticles of the refractory metal (e.g., titanium (Ti), zirconium(Zr), hafnium, (Hf), niobium (Nb), tantalum (Ta), tungsten (W) orsilicon (Si)), each of which is surrounded by a layer of molecules ofthe surfactant. For example, in the present exemplary embodiment, inwhich, the precursor is TiCl, the reactant is sodium naphthalene, andthe surfactant is n-hexylamine (CH₃(CH₂)₅NH₂), the reaction proceeds asfollows. Initially, the reactant and the refractory metal precursorreact to free elemental Ti from the precursor. The sodium, chloride(NaCl) precipitates out of the solution, while the free atoms ofTi_(metal) rapidly coalesce to form Ti nanoparticles. The free electronsin the NH₂ end of the n-hexylamine (CH₃(CH₂)₅NH₂) surfactant moleculesin the solvent are drawn to and form bonds with the dangling bonds(i.e., the unsaturated bonding orbitals) of the outermost Ti atoms inthe rapidly growing Ti nanoparticles, such that the surfactant moleculesform a protective barrier around the nanoparticles which prevents theirfurther growth. In this fashion, the reaction is halted before the Tinanoparticles have bad a chance to further coalesce into a larger mass.The promoter (e.g., naphthalene, anthracene, phenanthrene, etc.) doesnot take part in the reaction directly and can be reclaimed and recycledby vacuum sublimation, as illustrated in the generalized exemplaryequations set forth below:TiCl₄+4Na(promoter)→nano-Ti(surfactant)+4NaCl+4 promoterZRCl₄+4Na(promoter)→nano-Zr(surfactant)+4NaCl+4 promoterHfCl₄+4Na(promoter)→nano-Hf(surfactant)+4NaCl+4 promoterNbCl₅+5Na(promoter)→nano-Nb(surfactant)+5NaCl+5 promoterTaCl₅+5Na(promoter)→nano-Ta(surfactant)+5NaCl+5 promoterWCl₆+6Na(promoter)→nano-W(surfactant)+6NaCl+6 promoterSiCl₄+4Na(promoter)→nano-Si(surfactant)+4NaCl+4 promoter

While the foregoing equations illustrate reactions with stoichiometricamounts of a promoter, the scope of the present invention, is notlimited to such an arrangement. Rather, the invention has application toembodiments in which far less of the promoter is present during thereaction (e.g., less than 10% of the stoichiometric amount maysuccessfully be used).

According to one aspect of the present invention, the reactant may be analkali or alkaline earth metal dissolved in an ether, optionallyincluding a promoter such as naphthalene (C₁₀H₃) anthracene (C₁₄H₁₀) orphenanthrene (C₁₄H₁₀). For example, the reactant may be a sodiumnaphthalene mixture formed according to the following method. First, 2 gof Na (87 mmol) cut into small pieces for faster reaction (commerciallyavailable Na sand can be used as well) are added, to a dry reactioncontainer filled with an inert gas atmosphere containing 12 g ofnaphthalene (93.6 mmol). The reaction container is heated while themixture is stirred, and the temperature is monitored via a thermocouple.The temperature should increase from room temperature to 130° C. inabout 20 minutes. Naphthalene starts subliming around 120° C. andsometimes some of it will melt during this procedure. When 130° C. isreached, and after the naphthalene is sublimed, the mixture is chilled,(e.g., with a dry ice/DI water bath) to 0° C., and 100 ml dry THF isadded to the mixture. The mixture will almost immediately turn a verydark forest green, color, indicating the successful formation of thesodium naphthalene complex. The reaction is kept cold by adding dry iceand water to the cooling bath in order to prevent the sodium complexfrom decomposing hack into elemental sodium. After 1-2 hours, the sodiumwill have dissolved, depending on the size of the Na pieces added andhow well the procedure has been carried out. It is important to haveabsolutely dry reaction containers and chemicals, since the sodium is inan extremely reactive state and will react immediately with any moistureor oxygen or any other reactive chemical present.

According to another aspect of the present invention, the reactant maybe a lithium naphthalene mixture formed according to the followingmethod, 0.7 g (101 mmol) of Li cut into small pieces for faster reactionare added to a dry reaction container filled with an inert gasatmosphere containing 12 g of naphthalene (93.6 mmol). The reactioncontainer is heated while the mixture is stirred, and the temperature ismonitored via a thermocouple. The temperature should increase from roomtemperature to 130° C. in about 20 minutes. Naphthalene starts sublimingaround 120° C. and sometimes some of it will melt during this procedure.When 130° C. is reached, and after the naphthalene, is sublimed, themixture is chilled (e.g., with a dry ice/DI water bath) to 0° C., and100 ml dry THF is added to the mixture. The mixture will almostimmediately turn a very dark forest green color, indicating thesuccessful formation of the lithium naphthalene complex. The reaction iskept cold by adding dry ice and water to the cooling bath in order toprevent the lithium complex from decomposing back into elementallithium. After 1-2 hours, the lithium will have dissolved, depending onthe size of the Li pieces added and how well the procedure has beencarried out. It is important to have absolutely dry reaction, containersand chemicals, since the lithium is in an extremely reactive state andwill react immediately with any moisture or oxygen or any other reactivechemical present.

According to another aspect of the present invention, the reactant maybe a potassium naphthalene mixture formed according to the followingmethod. 3.4 g (87 mmol) of K cut into small pieces for faster reactionare added to a dry reaction container filled with an inert gasatmosphere containing 12 g of naphthalene (93.6 mmol). The reactioncontainer is heated while the mixture is stirred, and the temperature ismonitored via a thermocouple. The temperature should increase from roomtemperature to 130° C. in about 20 minutes. Naphthalene starts sublimingaround 120° C. and sometimes some of it will melt during this procedure.When 130° C. is reached, and after the naphthalene is sublimed, themixture is chilled (e.g., with a dry ice/DI water bath) to 0° C., and100 ml dry THF is added to the mixture. The mixture will almostimmediately turn, a very dark forest green color, indicating thesuccessful formation of the potassium naphthalene complex. The reactionis kept cold by adding dry ice and water to the cooling bath in order toprevent the potassium complex from decomposing back into elemental K.Alter 1-2 hours, the potassium will have dissolved, depending on thesize of the K pieces added and how well the procedure has been carriedout. It is important to have absolutely dry reaction containers andchemicals, since the potassium is in an extremely reactive state andwill react immediately with any moisture or oxygen or any other reactivechemical present.

According to yet other aspects of the present invention, the reactantmay be a mixture of any one of lithium, potassium, sodium, magnesium orcalcium, together with naphthalene, anthracene or phenanthrene.

According to one aspect of the present invention, the order in which thereagents are combined is important for ensuring a narrow sizedistribution of refractory metal nanoparticles. For example, when boththe surfactant and the precursor are added to the solvent (andthoroughly distributed therein) before the reactant is added thereto,local differences in the concentration of the refractory metal precursorand the surfactant can be avoided. This equilibrium ensures that whenthe reactant is added, the refractory metal nanoparticles that formwill, term in a similar manner and achieve similar sizes.

Moreover, the speed with which the reactant is added to the reaction isimportant for ensuring a narrow size distribution of refractory metalnanoparticles, in accordance with one embodiment of the presentinvention. By slowly adding the reactant (e.g., at a rate of about 50 to60 drops per minute), local differences in the concentration of thereactant can similarly be avoided, to ensure that the refectory metalnanoparticles form in near-equilibrium conditions and achieve similarfinal sizes. Alternatively, by more quickly adding the reactant (e.g.,at a rate greater than 100 drops per minute), a larger particle sizedistribution can be achieved (e.g., due to the local, concentrationdifferences that occur). In accordance with one embodiment of thepresent invention, reactant can be slowly added until nearly all of theprecursor has been consumed, at which time a surplus of reactant (i.e.,more than is needed to react with all of the refractory metal precursor)is quickly added to ensure that all the remaining precursor is reactedwith. This approach offers the advantage of ensuring that nearly all ofthe nanoparticles will achieve a similar size, while also ensuring thatall of the precursor is consumed.

While in the foregoing exemplary embodiment, the surfactant used toprevent further nanoparticle growth is n-hexylamine, the scope of thepresent invention is not limited to this arrangement. Rather, as will beapparent to one of skill in the art, any one of a number of polarsurfactant molecules with free electrons or electron pairs may be usedas a surfactant with refractory metal nanoparticles. Moreover, while inthe foregoing exemplary embodiment, only one surfactant was used, thepresent invention has application, to reactions in which multiplesurfactants are used to control the growth of nanoparticles. Forexample, the surfactant(s) used may be any one or more surfactantschosen from the illustrative list in Table 1, below:

TABLE 1 Surfactant Type Representative List Organic amine Pyridine(mono-, di- and tri-) Triethanol amine Diethylene triamine Ethylenediamine (C2 to C16) Hexyl-, nonyl-, Dodecyl amine (C6 to C16) Organicamine halide salt Hexyl- nonyl-, Hexadecyl ammonium (mono-, di-, tri-and tetra-) chloride (C6 to C16) Hexyl- nonyl-, Hexadecyl ammoniumboride (C6 to C16) Organic alcohol Octanol, decanol, dodecanol (C6 toC16) Organic acid C2 to C16: Acetic acid, Hexanoic acid Oleic acid(cis-9-octadecenoic acid) Organic phosphines/ Tri-phenyl phosphinephospine oxides Tri-ethyl phosphine oxide C1 to C10 Organic nitrileAcetonitrile Benzonitrile C2 to C16

For example, in one embodiment of the present invention, a mixture of anorganic phosphine/phosphine oxide, an organic amine, and an organicamine halide salt may be used to provide improved particle size controlfor a particular reaction (as one surfactant may be preferentiallycoordinating to the metal precursor, while another surfactant maypreferentially coordinate with the elemental metal, and yet a thirdsurfactant may preferentially coordinate with the nanoparticle).

In accordance with one aspect of the present invention, as some strongreducing agents (e.g., Li, K, Na, Me. Ca, etc.) can and do react withorganic nitriles, they should therefore not be used during the initial,nanoparticle formation, but these can later be exchanged for, e.g.,saturated amines.

In accordance with one important aspect of the present invention, thelength of the carbon chains and number thereof on the surfactantmolecule play an important role in determining the amount, of protectiongiven the nanoparticle. For straight, single hydrocarbon chain systems,chains shorter than C6 do not bond sufficiently with the nanoparticle toprotect it, as little thermal energy is required to cause thesemolecules to come off. For chains longer than C16, the surfactantmaterial is increasingly difficult to remove, which will make formingrefractory metal coatings increasingly difficult, as is described ingreater detail below. In general, amines bond more strongly torefractory metal nanoparticles than do alcohols, as the former exhibithigher Lewis Base strength, due to their ability to establish a highercovalent bond character.

While the foregoing exemplary embodiment has been described withreference to TiCl₄ reacting with sodium naphthalene, the scope of thepresent, invention is not limited to this particular precursor and theseparticular reactants. Rather, the present invention has application toreactions for forming refractory nanoparticles which utilize anyrefractory metal, precursor and any reactant capable of freeingelemental refractory metals from their precursors.

According to another embodiment of the present invention, two or morerefractory metal precursors may be reacted in a single reaction, toprovide mixed metal systems and alloys. For example, by reacting a metalprecursor and a silicon precursor, metal silicides may be formed. Inaddition, it is possible to produce mixtures that do not form alloys orcompositions that do not like to mix in a particular ratio, producingthereby a two phase system with nano-scale distribution, for use insolar cell systems or nano-scale ceramic glass-like metal systems.

FIG. 2 is a flowchart illustrating a method for manufacturing refractorymetal nanoparticles in accordance with another embodiment of the presentinvention. In step 201, a poly-ether solvent, such as monoglymeCH₃—O—CH₂CH₂—O—CH₃, diglyme CH₃—O—(CH₂CH₂—O)₂—CH₃, triglymeCH₃—O—(CH₂CH₂—O)₃—CH₃, or any other glyme characterised by the chemicalformula R—O—(CH₂CH₂—O)_(x)—R, where x is a positive integer and R is amethyl group (CH₃), an ethyl group (C₂H₅), a propyl group (C₃H₇), or abutyl group (C₄H₆), is provided. In step 202, a refractory metalprecursor and a reactant are reacted in the poly-ether solvent to freerefractory metal atoms from the precursor. The byproducts of thisreaction are precipitated out of solution or boiled off. In step 203,the refractory metal particles are combined in the poly-ether solvent toform a refractory metal nanoparticle. In step 204, the refractory metalnanoparticle is surrounded, in the poly-ether solvent, with a layer ofsurfactant molecules. The surfactant molecules, which are provided inthe solvent at the beginning of the reaction, may be one or more ofn-hexylamine (CH₃(CH₂)₅NH₂), n-nonylamine (CH₃(CH₂)₈NH₂, n-dodecylamine(CH₃(CH₂)₁₁NH₂), or any other amine or mixture of amines characterisedby the chemical formula CH₃(CH₂)_(x)NH₂, where x is a positive integer.

Turning to FIG. 3, a reactor system used in the manufacture ofrefractory metal nanoparticles is illustrated in accordance with oneembodiment, of the present invention. Reactor system 300 includes acontinuous stirred-tank reactor 301, into which reagents 306 areprovided via a syringe or an addition funnel 302. An impeller 307 stirsthe reagents 306 to ensure thorough mixing thereof (e.g., to optimizethe particle size distribution of the refractory metal nanoparticles). Acondenser 303 allows gases created in the chemical reactions occurringin reactor 301 (e.g., hydrogen in the present exemplary embodiment) toescape through outlet 305, while coolant which flows through ports 304through condenser 303 cools more volatile species (such as thesurfactant or the solvent) and allows them to trickle back down alongOre corkscrew-shaped path in condenser 303 into reactor 301. Athermometer 308 is used to track the temperature of the reagents 306dining the chemical reaction. System 300 may further, include a heatsource (not illustrated) to increase the temperature of the reaction,and thereby control the size and size distribution of the refractorymetal nanoparticles, as described in greater detail below.

According to one aspect of the present invention, reactor 301 can beheated (or cooled) to control the temperature at which the reactionstherein take place. The duration for which heat is applied provides amechanism for ensuring even distribution of the reagents during thereaction and thorough mixing thereof so that the size distribution ofthe refractory metal nanoparticles can be narrowed. For example, inaccordance with one aspect of the present invention, reactor 301 isheated lot about 90 minutes after combining the solvent, refractory,metal precursor and surfactant therein, to ensure an even distributionthereof.

According to another aspect of the present invention, the concentrationof surfactant in the reaction can similarly modify the resultant sizeand size distribution of refractory metal nanoparticles. With higherconcentrations of surfactant, the refractory metal nanoparticles aremore likely to encounter and bond with surfactant molecules early intheir growth, resulting in both smaller nanoparticles, and a smallerdistribution of particle sizes.

While in the foregoing exemplary embodiments, the refractory metalnanoparticles have been described with respect to particular sizes, thescope of the present invention is not limited to these particulararrangements. For example, by reducing the concentration of surfactantin the reaction or increasing the speed with which the reactant is addedthereto, refractory metal nanoparticles larger than 100 nm may bemanufactured. Alternatively, by increasing the concentration ofsurfactant in the reaction, nanoparticles as small as 4 nm nay bemanufactured. As will be apparent to one of skill, in the art, thepresent invention has application to the manufacture of refractory metalnanoparticles of nearly any size.

In accordance with one exemplary experimental embodiment of the presentinvention, a mixture including 4.1 g (20 mmol) of TiCl₄ 20 ml of eitherTHF or triglyme and 80 mmol of dodecyl amine is provided. To thismixture, one of the above mixtures containing solubilized alkaline oralkaline earth metal with a tour-fold excess is slowly added via anaddition funnel, to produce four equivalents of NaCl and titaniumnanoparticles, which appear as black powder. For the success of thereaction, it is important to have dry reaction containers and chemicals,or an oxide will be formed in lieu of the metal nanoparticles. The Tiparticle size is controlled by both the amount of the amine surfactantin the mixture and the temperature at which the reaction with thereagent is carried out. With these particular reagents, the reactioncommences very rapidly and can be carried out at room temperature (e.g.,about 20° C.).

In accordance with another exemplary experimental embodiment of thepresent invention, a mixture including 5.75 g (14.5 mmol) of WCl₆, 50 mlof either THF or triglyme and 50 mmol of dodecyl amine is provided. Tothis mixture, one of the above mixtures containing solubilized alkalineor alkaline earth metal with a four-fold excess is slowly added via anaddition funnel, to produce tour equivalents of NaCl and tungstennanoparticles (black powder). For the success of the reaction, it isimportant to have dry reaction, containers and chemicals, or an oxidewill be formed in lieu of the metal nanoparticles. The W particle sizeis controlled by both the amount of the amine surfactant in the mixtureand the temperature at which the reaction with the reagent is carriedout. With these particular reagents, the reaction, commences veryrapidly and can be carried out at room temperature (e.g., about 20° C.).

In accordance with yet another exemplary experimental embodiment of thepresent invention, a mixture Including 4.1 g (20 mmol) of SiCl₄, with 20ml of either THF or triglyme and 80 mmol of dodecyl amine is provided.To this mixture, one of the above mixtures containing solubilizedalkaline or alkaline earth metal with a four-fold excess is slowly addedvia addition funnel, to produce four equivalents of NaCl and siliconnanoparticles (black powder). The Si particle size is controlled by boththe amount of the amine surfactant in the mixture and tire temperatureat which the reaction with the reagent is carried out. With theseparticular reagents, the reaction, commences very rapidly and can becarried out at room temperature (e.g., about 20° C.).

FIG. 4 illustrates a single refractory metal nanoparticle in greaterdetail. In accordance with one embodiment of the present invention.Refractory metal nanoparticle 400 includes a plurality of refractorymetal atoms 401 tightly bonded together. Surrounding atoms 401 is alayer of surfactant molecules 402, in this case, n-hexylamine(CH₃(CH₂)₅NH₂). The NH₂ end of each, n-hexylamine molecule has bondedwith the dangling bonds (i.e., the unsaturated bonding orbitals) of theoutermost refractory metal atoms 401 in refractory metal nanoparticle400 to form a protective barrier around nanoparticle 400. While FIG. 4illustrates a single particle in cross-section, showing only a ring ofsurfactant molecules at the periphery of the nanoparticle, an actualnanoparticle has a three-dimensional shell of surfactant molecules 402surrounding the refractory metal atoms 401 on all sides.

In accordance with one aspect of the present invention, surfactantmolecules 402 need not be the same surfactant molecules used to stoprefractory metal nanoparticle 400 from growing during the manufacturethereof. Rather, as will be immediately understood by one of skill inthe art, a simple ligand exchange may be used to replace some or all ofthe surfactant molecules which were used in the manufacture ofrefractory metal nanoparticle 400.

FIG. 5 illustrates a refractory metal nanoparticle mixture in accordancewith one embodiment of the present invention. Nanoparticle mixture 500includes a solvent 501, in which are disposed a plurality of refractorymetal nanoparticles 502. The solvent 501 need not be the same poly-ethersolvent used in the manufacturing process of refractory metalnanoparticles 502. Rather, different solvents may be used innanoparticle mixture 500 depending upon whether nanoparticle mixture 500is to be used to coat a surface, as described in greater detail below,or whether nanoparticle mixture 500 is being stored. For example, in thepresent exemplary embodiment of the present invention, solvent 501 ishexane CH₃(CH₂)₄CH₃, a solvent with a low boiling point (˜69° C.),suitable for use in the refractory metal nanoparticle coating processesdescribed in greater detail below. Each nanoparticle 502 is surroundedby a layer of surfactant molecules which form a protective barrieraround the nanoparticle, preventing it from chemically reacting withother substances, such as air or moisture. This protective layer ofsurfactants around each nanoparticle 502 allows mixture 500 to behandled with relative safety (e.g., as the pyrophoricity of thenanoparticles may be negated thereby).

In accordance with various aspects of the present invention, dependingupon, their size, nanoparticles 502 may either be dissolved in solvent501, or may alternately form a slurry therewith. For example, in hexane,nanoparticles smaller than 10 nm will dissolve, while those larger thanabout 10 nm will not. Alternatively, in solvents such as xylene ortoluene, larger nanoparticles will still be soluble. According to anadditional aspect of the present invention, when the size distributionof nanoparticles 502 is large enough (or if more than one narrow sizerange of nanoparticles is present), some nanoparticles may be dissolvedin the solvent, while others form a slurry therewith.

The ability of some solvents to dissolve smaller nanoparticles thanothers can be exploited to separate nanoparticles of different sizes. Inaccordance with one embodiment of the present invention. For example, byintroducing the nanoparticles into a hexane solvent nanoparticles largerthan 10 nm (i.e., those that do not dissolve in hexane) can be separatedfrom nanoparticles smaller than 10 nm (i.e., those that do dissolve inhexane). These larger particles can then be introduced into a differentsolvent, such as xylene or toluene, to again separate smaller and largerparticles (depending upon their solubility in this solvent). A thirdfraction of the nanoparticles can similarly be separated out by size inyet another solvent such as isopropyl alcohol (“IPA”). Nanoparticles(and agglomerates thereof) which are larger than about 100 nm will notdissolve well in any known organic solvent with low polarity.

In accordance with one aspect of the present invention, refractory metalnanoparticle mixture 500 may include refractory metal nanoparticles 502of a variety of sizes and of different compositions. For example,nanoparticles 502 may have a single, continuous particle sizedistribution, as a result of all the nanoparticles being created in asingle reaction. Alternatively, nanoparticles 502 may have multiplenon-continuous particle size distributions (e.g., as a result of mixingnanoparticles produced in separate reactions, or of separatecompositions), where some of the nanoparticles are smaller (e.g. between4 nm and 10 nm), and the remainder of the panicles are larger (e.g.,between 25 nm and 100 nm). This arrangement may be particularlydesirable for increasing the volumetric packing efficiency ofnanoparticles in a refractory metal coating, as discussed in greaterdetail below, it will be immediately apparent to one of skill in the artthat the foregoing embodiments are merely exemplary, and that thepresent invention has application to refractory metal nanoparticlemixtures with any size refractory metal nanoparticles of any compositionand with any panicle size distribution.

II. Refractory Metal Carbide Nanoparticles

FIG. 6 is a flow chart illustrating an exemplary method by whichrefractory metal carbide nanoparticles may be manufactured in accordancewith one embodiment of the present invention. The method begins withstep 601, in which a refractory metal halide is provided. In step 602,the refractory metal halide is treated with a reagent to provide arefractory metal alkyl precursor, such as, for example, Ti(CH₃)₄.According to various aspects of the present invention, the reagent maybe a Grignard reagent (e.g., an alkyl- or aryl-magnesium halides) or alithium organic reagent. All three are readily available or easilyprepared for a wide variety of alkyl groups, and have the additionaladvantage that Li and Mg are highly electropositive and thus usuallycause the equilibrium of the reaction to lie to the right. When alithium organic reagent is used, such as LiCH₃, for example, theresultant lithium chloride precipitates and helps to force the reactionto the right. Moreover, such lithium organic reagents are readilyavailable for many compounds and are easily synthesized from an organichalide (e.g. ICH₃) and lithium metal. Some exemplary equationsrepresenting the treatment of step 602 are set forth below, for theprovision of titanium, zirconium, and tungsten alkyl precursors:TiCl₄+4LiCH₃→Ti(CH₃)₄+4LiCl (preferably conducted below −40° C.)ZrCl₄4LiCH₃→Zr(CH₃)₄+4LiCl (preferably conducted below −15° C.)WCl₆+6LiCH₃→W(CH₃)₆+6LiCl (m.p. 30° C.

The refractory metal alkyl precursor thus formed includes the refractorymetal and one or more alkyl groups. In step 603, the refractory metal,alkyl is heated in the presence of a surfactant. In order to decomposethe refractory metal alkyl precursor into a refractory metal carbidenanoparticle surrounded by a layer of molecules of the surfactant. Anexemplary equation representing the heating of a titanium alkylprecursor is set forth below:Ti(CH₃)₄+heat→TiC+3CH₄

According to one aspect the “heat” In the above equation may be morethan about −40° C. This low temperature allows for good particle sizecontrol, due to the reduced kinetic activity at that temperature.

According to another aspect of the present invention, metalorganiccompounds that allow for β-elimination may not be desirable, as thesecan lead to the formation of metal hydrides and pure metals, rather branthe desired carbide, and/or may provide undesired sub-stoichiometriccarbon content. For example, Ti(C₂H₅)₄ may not be a desirable precursor,as this precursor would be impracticably difficult to decompose into acarbide, due to the β-elimination mechanism and its low stability (e.g.,requiring temperatures below −80° C.):Ti(C₂H₅)₄+heat→Ti+4C₂H₄+2H₂

In some instances, the decomposition may not proceed as cleanly asdesired, resulting in only a refractory metal carbide, but rather mayproduce a nonstoichiometric product mixture, thereby preventing thedesired control. This is dependent upon the refractory metal alkyprecursor chosen, as will be readily apparent to those of skill in theart. For example, Al(C₃H₇)₃ may be used effectively for aluminumchemical vapor deposition for microelectronics applications.

In a similar manner to that described above with reference to themanufacture of refractory metal nanoparticles, the heating of arefractory metal alkyl precursor should occur in the presence of asurfactant in order to halt the growth of the refractory metal carbidenanoparticles at a desired size or size range. Similar surfactantsand/or surfactant mixtures to those described above (e.g., in Table 1)may be used, in accordance with, various aspects of the presentinvention.

According to another embodiment of the present invention, the formingmetal nanoparticles can be “capped” with various organic groups (e.g.,one or more methyls, ethyls, butyls, etc) by using an otherwiseinsufficient amount of reducing agent, thereby leaving reactive halidegroups on the nanoparticles' surfaces. The reagents used may include aGrignard reagent (e.g., an alkyl- or aryl-magnesium halide) or a lithiumorganic reagent. Both are readily available or easily prepared for awide variety of alkyl groups, and have the additional advantage that Liand Mg are highly electropositve and thus usually cause the equilibriumof the reaction to lie to the right. When a lithium organic reagent isused, such as LiCH₃, for example, the resultant lithium chlorideprecipitates and helps to force the reaction to the right. Moreover,such lithium organic reagents are readily available for many compoundsand are easily synthesized from an organic halide (e.g. ICH₃) andlithium metal. Some exemplary equations representing the treatment areset forth below:_(nano)Hf₂₀₀₀Cl₁₀₀+100LiCH₃→_(nano)Hf₂₀₀₀(CH₃)₁₀₀+100LiCl (conduct below−15° C. because of thermal instability)_(nano)Ta₂₀₀₀Cl₁₀₀+100LiCH₃→_(nano)Ta₂₀₀₀(CH₃)₁₀₀+100LiCl (conduct below0° C. because of thermal instability)_(nano)W₂₀₀₀Cl₁₀₀+100LiCH₃→_(nano)W₂₀₀₀(CH₃)₁₀₀+100LiCl (roomtemperature stable)

The advantage of this, pathway is that the organic groups lend very goodoxidation protection and stop particle growth, since they bondcovalently to the nanoparticle surface and therefore particularly suitedfor the very reactive metals such as Zr, Hf, Nb, Ta, W and Si. Only atelevated temperature would the organic groups start to oxidize and burnoff.

According to another aspect of the present invention, these cappednanoparticles can be thermally decomposed directly into nanostructuredcarbide coatings or solid parts via moderate heating. Some exemplaryequations representing the treatment are set forth below:_(nano)Hf₂₀₀₀(CH₃)₁₀₀(surf)+heat→_(nano)HfC+H₂+volatiles_(nano)Ta₂₀₀₀(CH₃)₁₀₀(surf)+heat→_(nano)TaC+H₂+volatiles_(nano)W₂₀₀₀(CH₃)₁₀₀(surf)+heat→_(nano)WC+H₂+volatiles

According to one aspect of the present invention, the concentration ofsurfactant in the reaction can modify the resultant size and sizedistribution of refractory metal carbide nanoparticles. With higherconcentrations of surfactant, the refractory metal carbide nanoparticlesare more likely to encounter and bond with surfactant molecules early intheir growth, resulting in both smaller nanoparticles and a smallerdistribution of particle sizes.

In accordance with one aspect of the present invention, a refractorymetal carbide nanoparticle may be surrounded by surfactant moleculesdifferent from those used to stop the refractory metal, carbidenanoparticle from growing during the manufacture thereof via a simpleligand exchange.

In accordance with another aspect of the present invention, a refractorymetal carbide nanoparticle mixture may be provided, in which a pluralityof refractory metal carbide nanoparticles are disposed in a solvent.Different solvents may be used in the nanoparticle mixture, dependingupon whether nanoparticle mixture is to be used to coat a surface, asdescribed in greater detail below, or whether the nanoparticle mixtureis being stored. For example, in one exemplary embodiment of the presentinvention, a solvent with a low boiling point such as hexane (˜69° C.)may be used in live refractory metal carbide nanoparticle coatingprocesses described in greater detail below.

In accordance with various aspects of the present invention, dependingupon their size, the nanoparticles may either be dissolved in a solvent,or may alternately form a slurry therewith. For example, in hexane,nanoparticles smaller than 10 nm will dissolve, while those larger thanabout 10 nm will not. Alternatively, in solvents such as xylene ortoluene, larger nanoparticles will still be soluble. According to anadditional aspect of the present invention, when the size distributionof nanoparticles is large enough (or if more than one narrow size rangeof nanoparticles is present), some nanoparticles may be dissolved in thesolvent, while others form a slurry therewith.

The ability of some solvents to dissolve smaller nanoparticles thanothers can be exploited to separate nanoparticles of different sizes, inaccordance with one embodiment of the present invention. For example, byintroducing the nanoparticles into a hexane solvent nanoparticles largerthan 10 nm (i.e., those that do not dissolve in hexane) can be separatedfrom nanoparticles smaller than 10 nm (i.e., those that do dissolve inhexane). These larger particles can then be introduced into a different,solvent, such as xylene or toluene, to again separate smaller and largerparticles (depending upon their solubility in this solvent). A thirdfraction of the nanoparticles can similarly be separated, out by size inyet another solvent such as isopropyl alcohol (“IPA”). Nanoparticles(and agglomerates thereof) which are larger than about 100 nm will notdissolve well in any known organic solvent with low polarity.

In accordance with one aspect of the present invention, a refractorymetal carbide nanoparticle mixture may include refractory metal carbidenanoparticles of a variety of sizes and of different, compositions.Moreover, a mixture may include both refractory metal nanoparticles andrefractory metal carbide nanoparticles. The nanoparticles may have asingle, continuous particle size distribution, as a result of all thenanoparticles being created in a single reaction. Alternatively, thenanoparticles may have multiple non-continuous particle sizedistributions (e.g., as a result of mixing nanoparticles produced inseparate reactions, or of separate compositions), where some of thenanoparticles are smaller (e.g. between 4 nm and 10 nm), and theremainder of the particles are larger (e.g., between 25 nm and 100 nm).This arrangement may be particularly desirable for increasing thevolumetric packing efficiency of nanoparticles in a refractory metalcarbide coating, as discussed in greater detail below, it will beimmediately apparent to one of skill in the art that the foregoingembodiments are merely exemplary, and that the present, invention hasapplication to refractory metal carbide nanoparticle mixtures with anysize refractory metal carbide nanoparticles of any composition and withany particle size distribution.

III. Nanoparticle Coatings

In accordance with one aspect of the present invention, a refractorymetal or refractory metal carbide nanoparticle mixture can be “painted”onto surfaces to form thin coatings of refractory metals or refractorymetal carbides. This is accomplished, by disposing a refractory metal ora refractory metal carbide nanoparticle mixture onto a surface to becoated, and hearing the “painted” mixture with progressively highertemperatures to (1) boil off the solvent, (2) boil off the surfactantand (3) “fuse” adjacent nanoparticles together. This process isdescribed in greater detail below, with respect to FIG. 7.

FIG. 7 is a flowchart illustrating a method for forming a refractory,metal or refractory metal carbide coating in accordance with oneembodiment of the present, invention. The method begins with step 701,in which a refractory metal or refractory metal carbide nanoparticlemixture is provided. The nanoparticle mixture includes a solvent and aplurality of refractory metal or refractory metal carbide nanoparticles,each surrounded by a layer of surfactant molecules. In step 702, thenanoparticle mixture is disposed on a surface to be coated. Inaccordance with one important aspect of the present invention, thecoating process does not require the surface to be coated to withstandthe very high temperatures associated with most metallurgical approachesto coating a surface with refractory metals or their carbides (althoughmany coated surfaces will have application in high-temperatureenvironments, and accordingly may be capable of withstanding suchtemperatures). For example, the surface to be coated may be acarbon-based material, such as graphite or a carbon/carbon (“C/C”)composite, which can withstand the low temperature coating process(e.g., which may occur at less than 20% of the bulk melting point of acarbide or refractory material used to coat it). In step 703, the coatedsurface is heated to a first temperature to evaporate the solvent fromthe nanoparticle mixture. Leaving the coated refractory metal orrefractory metal carbide nanoparticles arranged in a lattice on thesurface. In the present exemplary embodiment, the first temperature ischosen to be a temperature sufficiently high to evaporate the solvent,but not high enough to evaporate the surfactant layer around in eachmolecule. For example, in an embodiment in which the solvent in thenanoparticle mixture is hexane, and in which the surfactant moleculesare hexylamine molecules, the first temperature may be between 125° C.and 175° C.

Turning briefly to FIG. 8, the first heating step 703 is illustrated ingreater detail in accordance with one embodiment of the presentinvention. In FIG. 8, the nanoparticle mixture 801, which has beendisposed on surface 802, is beginning to evaporate, leaving refractorymetal or refractory metal carbide nanoparticles 803 arranged on surface802.

Returning to FIG. 7, the process continues with, step 704, in which thecoated surface is heated to a second temperature to remove thesurfactant layers from around the nanoparticles. In accordance with thepresent exemplary embodiment of the invention, in which the surfactantis hexylamine, the second temperature is between about 130° C. and 150°C. In removing the surfactant molecules, this second heating stepeffects a volumetric contraction of the lattice of refractory metal orrefractory metal carbide nanoparticles.

Depending upon the surfactant (or combination of surfactants) present,in the nanoparticle mixture, the volume of the coated refractory metalor refractory metal carbide nanoparticles may be significantly largerthan the uncoated nanoparticles left after second heating step 704. Forexample, in accordance with, the present exemplary embodiment, in whichthe (single) surfactant used is hexylamine, the volume of thenanoparticles contracts by about 1.7% after the second heating step.Accordingly, in choosing a surfactant (or combination of surfactants) touse in preparing a nanoparticle mixture, those which provide a lesservolumetric contraction during this step may be desirable to reducecracking of the resultant coating, in accordance with one aspect of thepresent invention. This contraction is illustrated in greater detail inFIGS. 9A and 9B, in accordance with one embodiment of the presentinvention.

FIG. 9A illustrates an orderly lattice of surfactant-covered refractorymetal or refractory metal carbide nanoparticles 901 disposed on asurface 902. As can be seen with reference to FIG. 9A, the surfactantmolecules 903 space the cores of the nanoparticles a significantdistance apart. Once these particles are heated to the secondtemperature of step 704, the surfactant molecules 903 are removed, andnanoparticles 901 contract in their absence, as illustrated in FIG. 9B.Depending upon the surfactant or surfactants used, this contraction maybe significant enough to form cracks in the coating on surface 902. Forexample, the removal of longer surfactant molecules (e.g., with carbonchains longer than C16) may cause the refractory metal or refractorymetal carbide nanoparticles to contract to such an extent that therefractory metal or refractory metal, carbide coating spalls and flakesoff of surface 902.

While FIGS. 9A and 9B illustrate an embodiment of the present inventionin which nanoparticles 901 are all approximately the same size (i.e.,they have a very narrow particle size distribution), the scope of thepresent invention is not limited to such an arrangement. Rather, thepresent invention has application to nanoparticle coatings in whichnanoparticles of different sizes and/or compositions are used to improvethe packing efficiency thereof. For example, in accordance with oneembodiment of the present invention, nanoparticles of two approximatesizes may be provided in a nanoparticle mixture, where the smallernanoparticles are approximately 10% of the volume of the largerparticles, to provide greater spherical packing efficiency (wherein thesmaller particles occupy the interstitial spaces between the largerparticles in the lattice). The smaller nanoparticles may have the samecomposition as the larger particles, or they may be a differentcomposition (e.g., including a different refractory metal or refractorymetal, carbide). Other arrangements with different particle sizedistributions may also be provided, to further improve packingefficiency and crack resilience, within the scope of the presentinvention.

While in the foregoing exemplary embodiment, the evaporation of thesolvent has been described as occurring, prior to the removal of thesurfactant, the scope of the present invention is not limited to such anarrangement. Rather, depending upon the surfactants and solvents used inthe nanoparticle coating process, one or more surfactants may be removedprior to, or at the same time as, one or more of the surfactants used.Thus, in accordance with various embodiments of the present inventionsteps 703 and 704 may occur in any order, or may be combined in to asingle step, in which the first and second temperature are one and thesame.

Returning to FIG. 7, the process continues with step 705, in which thecoated surface is heated to a third temperature to bond thenanoparticles together to Form a coating on the surface. In accordancewith the present exemplary embodiment of the invention, the thirdtemperature is between, about 550° C. and 1000° C. (depending upon themelting point of the specific material). This temperature “fuses” orbonds the adjacent nanoparticles to each other, to form a coherentcoating of the refractory metal or refractory metal carbide on thesubstrate. This bonding is illustrated in FIG. 9C, in whichnanoparticles 901 have bonded as a result of the application of thethird temperature.

The foregoing temperatures used to form the refractory metal orrefractory metal carbide coating are well below those necessary to formrefractory metal or refractory metal carbide coatings using otherapproaches. Accordingly, an important advantage of the present inventionis the ability to form refractory metal or refractory metal carbidecoatings on materials not previously capable of being so coated. Forexample, in accordance with various embodiments of the presentinvention, the foregoing method, can be used to coat carbon-basedmaterials (e.g., graphite, carbon/carbon composites) and othertemperature sensitive materials (e.g., materials that would melt,oxidize, or otherwise not withstand temperatures above 800° C. Ofcourse, as will be understood by those of skill in the art, the presentinvention, has application to coating a wide range of materials thatmight enjoy the benefits of a refractory metal or refractory metalcarbide coating.

Additionally, the simplicity of “painting” on a refractory metal orrefractory metal carbide nanoparticle mixture allows shapes notpreviously capable of being coated with refractory metal or refractorymetal carbides to be coated using the foregoing methods. In accordancewith yet other embodiments of the present invention. For example,internal surfaces of complex shapes (e.g., the inner diameter of tubesor nozzles), as well as high-aspect ratio surfaces, cap be coated with arefectory metal or refractory metal carbide nanoparticle coating, inaccordance with various embodiments of the present invention.

According to another embodiment of the present invention, a refractorymetal coating may be formed from refractory metal nanoparticles (asopposed to refractory metal carbide nanoparticles). One such exemplarymethod is set forth in FIG. 10. In step 1001, a refractory metalnanoparticle mixture including refractory metal nanoparticles in asolvent, is provided on a surface to be coated. The refractory metalnanoparticle mixture includes a non-volatile surfactant, which may beprovided in layers around the refractory metal nanoparticles by ligandexchange (e.g., exchanging the non-volatile surfactant for thesurfactant which originally halted the growth of the nanoparticle, asset forth in greater detail above), or which may be provided in amixture with the refractory metal nanoparticles (i.e., in addition tothe surfactant which surrounds the nanoparticles). Appropriatesurfactants include, for example, long chain organics (e.g., with morethan 10 carbon atoms and a functional group at one or both ends), suchas amines, acids, phosphines, acetylactonates, thioethers or thiols.Shorter surfactants may be used as well, when another carbon, source isdispersed with the metal nanoparticles. That carbon source may be, forexample, long chain hydrocarbon (e.g., paraffin wax). Using a secondaryhydrocarbon source helps to create a reducing atmosphere that preventsoxidation and/or the incorporation of oxygen into the coating.

In step 1002, the refractory metal nanoparticle mixture is heated to afirst temperature to evaporate the solvent and leave the plurality ofrefractory metal nanoparticles surrounded by surfactant molecules on thesurface, in step 1003, the refractory metal nanoparticles and thesurfactant molecules are heated to a second temperature to decompose,the surfactant molecules, and to react the plurality of refractory metalnanoparticles with carbon from the decomposed surfactant to provide arefractory metal carbide coating on the surface. Due to the highreactivity of the nano-particles, this process can take place at verylow temperatures (i.e., much lower than standard carbide processes). Ageneralized equation illustrating this process is set forth below, where“Nano-M” represents a nanoparticle of refractory metal:Nano-M(surfactant)+heat→Nano-MC+H₂+volatiles

For example, in accordance with one exemplary aspect of the presentinvention, the nanoparticle of interest may be titanium. Accordingly, adecomposition reaction for this nanoparticle may occur as set forth inthe following equation:Nano-Ti₁₈(octadecylamine)+heat→(Nano-TiC)₁₈+18H₂+NH₃

According to one aspect of the present invention, the foregoing methodmay be practiced with the surface to be coated and the nanoparticlemixture disposed in a chamber filled with inert gas, in order to preventthe surfactant from escaping from the nanoparticle mixture. This may bedone when the surfactant which surrounds the nanoparticles functions asthe carbon source for the reaction. According to yet another embodiment,however, if another non-volatile carbon source is added, the reactionmay occur under atmospheric conditions.

For example, if the surfactant from which the carbon for the carbide isobtained comes not from a layer of octadecylamine surrounding the Tinanoparticle, but rather comes from a secondary carbon source, such asoctadecane, the decomposition reaction may occur as set forth in thefollowing equation:Nano-Ti₁₈+octadecane+heat→(nano-TiC)₁₈18H₂

According to various aspects of the present invention, the surfactantsin the above reaction may include one or more amines such as aliphaticamines, w-butyl amines, pentyl, hexyl, octyl, etc. with one, two orthree aliphatic chains attached. The surfactants in the above reactionmay also include one or more phosphines or phosphineoxides with multiple(e.g., three) organic chains, such as trioctylphosphineoxide(O═P(C₈H₁₇)₃). It should be noted that phosphines or phosphineoxideswith one or more hydrogen atoms on the phosphorous may be undesirablyreactive and dangerously toxic.

The foregoing exemplary organic compounds are known to bond wed to mosttransition metal centers and have been shown to stabilize differentnanoparticles as well. The early electropositive transition metals suchas Zr, Hf, Nb, Ta bond more strongly to oxide functional groups thanamines or phosphines. Therefore, phosphine oxides and organic acids arethe preferred ligands for these metals as well as long chain alcoholssuch as octanol dodecanol, etc. Ti, W and Si exhibit a relative highaffinity to nitrogen (e.g., Ti burns in nitrogen, atmosphere to TiN) andtherefore amines may be a good choke for these metals.

IV. Formation of Bulk Parts

According to another embodiment of the present invention, refractorymetal, or refractory metal carbide nanoparticle mixtures may be used toharm fully dense components via sintering or hot pressing. For example,FIG. 11 is a flow chart illustrating a method for forming refractorymetal or refractory metal carbide components in accordance with oneembodiment of the present invention.

The method begins with step 1101, in which a refractory metal, orrefractory metal carbide nanoparticle mixture is provided in a mold. Themixture includes a solvent and a plurality of refractory metal (orcarbide) nanoparticles, each of the nanoparticles being surrounded by alayer of surfactant molecules. In step 1102, the refractory metalnanoparticle mixture is sintered to consolidate the refractory metalcomponent in the shape of the mold. According to various aspects of thepresent invention, the sintering may alternately comprise spark plasmasintering (SPS) or field assisted sintering (FAST). FAST uses directresistive heating with a high current (e.g., hundreds of amps, dependingon the electrical resistivity of the substrate) and a low voltage (e.g.,less than 25 V) to rapidly heat the mixture in the mold, with verymoderate pressure applied. This technique is very fast, taking mereminutes, but can nevertheless provide fully dense parts.

According to another embodiment, a refractory metal carbide componentcan be manufactured from refractory metal nanoparticles (as opposed torefractory metal carbide nanoparticles). One such exemplary method isset forth in FIG. 12. The method begins with step 1201, in which arefractory metal nanoparticle mixture is disposed in a mold. Therefractory metal nanoparticle mixture includes a non-volatilesurfactant, which may be provided in layers around the refractory metalnanoparticles by ligand exchange (e.g., exchanging the non-volatilesurfactant for the surfactant which originally halted the growth of thenanoparticle, as set forth in greater detail above), or which may beprovided in a mixture with the refractory metal nanoparticles (i.e., inaddition to the surfactant which surrounds the nanoparticles).Appropriate surfactants include, for example, long chain organics (e.g.,with more than 10 carbon atoms, and a functional group at one or bothends), such as amines, acids, phosphines, acetylactonates, thioethers orthiols. Shorter surfactants may be used as well when another carbonsource is dispersed with the metal nanoparticles. That carbon source maybe, for example, long chain hydrocarbon paraffin wax). Using a secondaryhydrocarbon, source helps to create a reducing atmosphere that preventsoxidation and/or the incorporation of oxygen into the component.

In step 1202, the refractory metal nanoparticle mixture is heated to afirst temperature to evaporate the solvent and leave the plurality ofrefractory metal nanoparticles surrounded by surfactant molecules in themold, in step 1203, the refractory metal nanoparticles and thesurfactant molecules are heated to a second temperature to decompose thesurfactant molecules and to react the plurality of refractory metalnanoparticles with carbon from the decomposed surfactant to provide arefractory metal carbide component.

Due to the high reactivity of the nano-particles, this process can takeplace at very low temperatures (i.e., much lower than standard carbideprocesses). A generalized equation illustrating this process is setforth below, where “Nano-M” represents a nanoparticle of refractorymetal:Nano-M(surfactant)+heat→Nano-MC+H₂+volatiles

For example. In accordance with one exemplary aspect of the presentinvention, the nanoparticle of interest may be titanium. Accordingly, adecomposition reaction for this nanoparticle may occur as set forth inthe following equation:Nano-ti₁₈(octadecylamine)+heat→(Nano-TiC)₁₈+18H₂+NH₃

According to one aspect of the present invention, the foregoing methodmay be practiced with the nanoparticle mixture and the mold disposed ina chamber filled with inert gas, in order to prevent the surfactant fromescaping from the nanoparticle mixture. This may be done when thesurfactant which surrounds the nanoparticles functions as the carbonsource for the reaction. According to yet another embodiment, however,if another non-volatile carbon source is added, the reaction may occurunder atmospheric conditions.

For example, if the surfactant from which the carbon for the carbide isobtained comes not from a layer of octadecylamine surrounding the Tinanoparticle, but rather comes from a secondary carbon source, such asoctadecane, the decomposition reaction may occur as set forth in thefollowing equation:Nano-Ti₁₈+octadecane+heat→(nano-TiC)₁₈+18H₂

According to various aspects of the present invention, the surfactantsin the above reaction may include one or more amines such as aliphaticamines, n-butyl amines, pentyl, hexyl, octyl, etc. with one, two orthree aliphatic chains attached. The surfactants in the above reactionmay also include one or more phosphines or phosphineoxides with multiple(e.g., three) organic chains, such as trioctylphosphineoxide(O═P(C₈H₁₇)₃. It should be noted that phosphines or phosphineoxideswith, one or more hydrogen atoms on the phosphorous may be undesirablyreactive and dangerously toxic.

The foregoing exemplary organic compounds are known to bond well to mosttransition metal centers and have been shown to stabilize differentnanoparticles as well. The early electropositive transition metals suchas Zr, Hf, Nb, Ta bond more strongly to oxide functional groups thanamines or phosphines. Therefore, phosphine oxides and organic acids arethe preferred ligands for these metals as well as long chain alcoholssuch as octanol, dodecanol, etc. Ti, W and Si exhibit a relative highaffinity to nitrogen (e.g., Ti burns in nitrogen atmosphere to TIN) andtherefore amines may be a good choice for these metals.

According to one aspect of the present invention, refractory metalcarbide coatings and components, whether manufactured from refractorymetal, nanoparticles or refractory metal carbide nanoparticles, enjoyenhanced deformation characteristics similar to those of metals. In thisregard, nanostructured carbide components and coatings formed accordingto one of the exemplary methods of the present invention exhibitdeformation before failure. This is advantageous, since many ceramicsare brittle below their ductile-to-brittle transition temperature. Thislow temperature brittleness of ten leads to low temperature failure. Anano-grained structure coupled with a stable grain size allows thematerial, to deform along its grain boundaries, as the nanoparticlestherein exhibit a large number of unsaturated or “dangling” bonds thatform a very strong connection to their neighboring particles. Duringdeformation, some bonds may be broken, but there are many others that donot break, keeping the coating or bulk part solid. This process issimilar to that which occurs in the deformation of metals.

The description of the invention is provided to enable any personskilled in the art to practice the various embodiments described herein.While the present invention has been particularly described withreference to the various figures and embodiments, it should, beunderstood that these are for illustration purposes only and should notbe taken as limiting the scope of the invention.

There may be many other ways to implement the invention. Variousfunctions and elements described herein may be partitioned differentlyfrom those shown without departing from the spirit and scope of theinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and generic principles definedherein may be applied to other embodiments. Thus, many changes andmodifications, may be made to the invention, by one having ordinaryskill in the art, without departing from the spirit and scope of theinvention.

A reference to an element in the singular is not intended to mean “oneand only one” unless specifically stated, but rather “one or more.” Theterm “some” refers to one or more. Underlined and/or italicized headingsand subheadings are used for convenience only, do not limit theinvention, and are not referred to in connection with the interpretationof the description of the invention. All structural and functionalequivalents to the elements of the various embodiments of the inventiondescribed throughout this disclosure that are known or later come to beknown to those of ordinary skill in the an are expressly incorporatedherein by reference and intended to be encompassed by the invention.Moreover, nothing disclosed herein is intended to be dedicated to thepublic regardless of whether such disclosure is explicitly recited inthe above description.

What is claimed is:
 1. A method for forming a refractory metal carbidecomponent, the method comprising the steps of: providing a refractorymetal nanoparticle mixture, the refractory metal nanoparticle mixtureincluding a solvent and a plurality of refractory metal nanoparticles,each of the plurality of refractory metal nanoparticles being surroundedby a layer of surfactant molecules in which the surfactant molecules arebonded to a surface of the refractory metal nanoparticles; disposing therefractory metal nanoparticle mixture in a mold; heating the refractorymetal nanoparticle mixture to a first temperature to evaporate thesolvent and leave the plurality of refractory metal nanoparticlessurrounded by the surfactant molecules and bonded thereto in the mold;and heating the plurality of refractory metal nanoparticles and thesurfactant molecules while in the mold to a second temperature tothermally decompose the surfactant molecules and to react the pluralityof refractory metal nanoparticles with carbon from the decomposedsurfactant molecules to provide a refractory metal carbide component. 2.The method according to claim 1, wherein the refractory metal carbidecomponent comprises a carbide of a refractory metal chosen from thegroup consisting of titanium (Ti), zirconium (Zr), hafnium (Hf), niobium(Nb), tantalum (Ta), tungsten (W), and silicon (Si).
 3. The methodaccording to claim 1, wherein the surfactant molecules comprise ahydrocarbon.
 4. The method according to claim 1, wherein heating to thesecond temperature occurs in an inert gas chamber.
 5. The method ofclaim 1, wherein the refractory metal nanoparticle mixture furthercomprises a secondary carbon source admixed with the plurality ofrefractory metal nanoparticles and the solvent.
 6. The method of claim5, wherein the secondary carbon source comprises a hydrocarbon.
 7. Themethod of claim 1, wherein the surfactant molecules comprise more than10 carbon atoms.